In the field of gas turbines, in particularly within the aircraft industry, there are many security regulations which have to be fulfilled. One of these is the so-called containment requirement which means that if any fragments, such as a turbine blade or a part thereof, for some reason come loose from the rotor of the turbine, such fragments must be retained within the housing enclosing the rotor to prevent the fragments from damaging other parts of for example an airplane. This in turns means that the wall of the housing has to be dimensioned to fulfill this requirement.
Although the invention will be described with respect to a gas turbine of an airplane engine, and in particular in the region of the turbine where a first part enclosing the rotor, such as a low-pressure turbine case, and a second part arranged downstream the first part with respect to the gas flow, such as a turbine rear frame, are interconnected, the invention is not in any way restricted to this particular embodiment, but may be used in many ways in turbines of different types.
In a low-pressure turbine, not only does the low-pressure turbine case have to fulfill the containment requirement, but also the regions upstream and downstream of the rotor have to fulfill this requirement. This means that the turbine rear frame downstream the rotor has to be dimensioned to fulfill the containment requirement.
According to known designs, the containment requirement is fulfilled by dimensioning the thickness of the wall of the structure which encloses the rotor, or in another way is arranged adjacent to the rotor so that any fragments which may come loose from the rotor are stopped by the wall.
However, in cases where material having a low energy reception capacity is used, for example in a turbine rear frame and/or if the rotor of for example the low-pressure turbine has high kinetic energy, a thick wall could cause problems. Such a thick wall of the turbine rear frame (or rather of the outer ring of the turbine rear frame, which outer ring is arranged close to the rotor) means that this ring becomes more rigid and has a slower thermal response. This implies a shorter service life of the construction in the region where the requisite struts and/or guide vanes are connected to the ring.
A known alternative solution is to design the low-pressure case with a length such that it extends along the whole region where the containment requirement has to be fulfilled. In this case, a disadvantage is that the outer ring of the front section of the turbine rear frame becomes very short for a given total length of the turbine, and this in turns leads to the same problem(s) discussed above.